A medical ultrasound system forms an image by acquiring individual ultrasound lines (or beams). The lines are adjacent to each other and cover the target area to be imaged. Each line is formed by transmitting an ultrasonic pulse in a particular spatial direction and receiving the reflected echoes from that direction. The spatial characteristics of the transmitted wave and the characteristics of the receive sensitivity determine the quality of the ultrasound image. It is desirable that the ultrasound line gather target information only from the intended direction and ignore targets at other directions. Unfortunately, this is an idealistic goal which is physically not achievable.
For ultrasound imaging, typically an array of piezoelectric transducer elements are driven with separate voltages. By selecting the time delay (or phase) and amplitude of the applied voltages, the individual transducer elements can be controlled to produce ultrasonic waves which combine to form a net ultrasonic wave that travels along a preferred vector direction and is focused at a selected point along the beam. The beamforming parameters of each of the firings may be varied to provide a change in maximum focus or otherwise change the content of the received data for each firing, e.g., by transmitting successive beams along the same scan line with the focal point of each beam being shifted relative to the focal point of the previous beam. When using a steered array, by changing the time delays and amplitudes of the applied voltages, the beam with its focal point can be moved in a plane to scan the object. When using a linear array, a focused beam directed normal to the array is scanned across the object by translating the aperture across the array from one firing to the next.
The same principles apply when the transducer probe is employed to receive the reflected sound in a receive mode. The voltages produced at the receiving transducer elements are summed so that the net signal is indicative of the ultrasound reflected from a single focal point in the object. As with the transmission mode, this focused reception of the ultrasonic energy is achieved by imparting separate time delay (and/or phase shifts) and gains to the signal from each receiving transducer element.
Medical ultrasound is a coherent imaging method. The spatial characteristics of the ultrasound line are determined by the transmit and receive apertures of the transducer and the frequency spectrum of the echo signal. The effective beam shape is the product of the transmit and receive diffraction patterns. Each diffraction pattern is characterized by its mainlobe width and sidelobe levels. In particular, the mainlobe width determines the image resolution, while the sidelobe levels affect the interference from targets outside the desired beam direction. To achieve good contrast resolution, a low sidelobe level is imperative. For example, when imaging an area of low echogenicity, the sidelobes of the beam can receive echoes from brighter structures surrounding the region of interest and contaminate the area of low echogenicity.
Aperture weighting is a way of reducing the sidelobe level in the diffraction pattern. Imaging systems with array transducers have employed aperture weighting for both transmit and receive apertures. In medical ultrasound, receive aperture weighting, due to its implementation simplicity, is more commonly applied than transmit weighting, which is presently limited to high-end systems because of the associated implementation cost and complexity.
A channel transmitter in a medical ultrasound system generates an excitation signal which is typically a square wave with two or three cycles. The waveform amplitude is controlled to achieve the weighting. While the square wave creates a significant amount of higher harmonics, this harmonic content does not affect beam formation because it is greatly suppressed by the bandpass characteristic of the transducer and the frequency-dependent attenuation of the propagation medium.
Both transmit and receive aperture weightings are commonly seen in medical ultrasound equipment. Receive aperture weighting is typically implemented as programmable attenuators and/or amplifiers on the signal paths of the transducer elements.
These functions can be well integrated into ASICs (application specific integrated circuits) or other custom chips. The additional product cost is not excessive and warrants the implementation of receive amplitude weighting even in lower-cost systems.
Transmit amplitude weighting can be employed with programmable high-voltage amplifiers, multiple power supplies or programmable current sources. All hardware functions are replicated for every transmit channel. Implementation of transmit aperture weighting is complex and costly and, therefore, not applied in low-end or midrange products. Consequently, need continues to exist for an economical way of implementing transmit aperture weighting in low-end and midrange ultrasound imaging systems.